I just finished Shutter Island by Dennis Lehane (soon to be a major motion picture starring Leonardo DiCaprio).

In some ways a pretty standard genre thriller (US Marshals go to a mental hospital on an isolated island to investigate a disappearance, things are more complicated than they seem, they get trapped on the island, etc.) but it turns out to have a twist.

SPOILER ALERT

It turns out to use another genre cliche, the unreliable narrator. But Lehane has walked into a classic trap - this kind of cliche is only appreciated by readers when done in as minimal a fashion as possible. The larger an author makes this, and the cleverer they are in its implementation, the more the readers will hate them for it.

When the twist is revealed at the end, that the main protagonist is actually a mental patient and the events of the story have been a combination of the staff putting on an act to help him work through his psychosis, and actual hallucinated episodes, everything that comes before it is undermined. The reader has spent an entire novel sympathizing with, or rooting for, the protagonist and the rug is completely pulled out from under them. It can't but leave an unsettled and unsatisfied feeling in many readers.